'Protovipers' and the Evolution of Snake Fangs

Changes leading to the evolution of the viperid fang involved lengthening of rear maxillary teeth and have usually been attributed to the adaptive value derived by providing for more effective venom injection. As an alternative interpretation, it is argued here that an initial factor favoring elongation was not venom injection but instead increased swallowing effectiveness. During swallowing, rear maxillary teeth are the first teeth of the maxilla to engage the prey upon each swallowing cycle. Further, they are geometrically positioned more favorably than other maxillary teeth so as to swing through a longer retraction arc and hence move the prey farther. Consequently, rear maxillary teeth might be expected to be the ones most likely modified in any dental adaptations that served swallowing. Elongation of a few teeth permits deeper penetration into the tissues and hence increases purchase of the jaws on the prey. These factors, it is argued, all favored the initial elongation of posterior maxillary teeth. However, once elongated to enhance swallowing effectiveness, they became a key feature preadapted for subsequent modification along several evolutionary pathways. One direction led toward the Xenodon and Heterodon-like snakes in which the manipulation function of the elongated teeth during swallowing was further enhanced. These snakes thus represent not 'protovipers' but simply a specialized form of snake in which the teeth aid prey manipulation during swallowing. A second line of evolution led to viperid snakes in which the emphasis shifts so that the elongated teeth now play a greater role in venom injection. Although the superficial impression would be that of two parallel lines of evolution involving rear maxillary tooth elongation, the selective forces acting in each line would be quite different. In one, the adaptations would serve to improve prey manipulation during swallowing while in the other to improve envenomation. Just as the mechanics of swallowing could have been the initial factor favoring tooth elongation, one might expect that oral gland secretions also served initially not a toxic function but as an aid in prey handling or swallowing.